The present invention relates to fuel assemblies for a light water nuclear reactor comprising in per se known manner a generally parallelepipedic unit incorporating a large number of elementary rods containing the sheathed fissile fuel material. Such an assembly comprises a skeleton having two solid end members braced by a bundle of hollow guide tubes in which are located control bars and, fixed at regular intervals on the aforementioned guide tubes, a series of spacing members, called grids, in the various square cross-section channels of which the fuel rods are arranged in easy-fit manner. These vertically juxtaposed assemblies forming the active core of a nuclear reactor are immersed in the pressurized cooling water serving to release the heat and conventionally have a length between 4 and 5 meters. The latter dimension, the considerable weight of each of the rods and the lightness of the spacing grids on an industrial scale leads to constructional difficulties, one of the most important of which being described hereinafter.
The connection between the guide tubes and the end members is often realized by screwing threaded members and the intermediate spacing members or grids are rigidly fixed to the guide tubes by welding or a mechanical deformation (expansion) of the guide tubes above and below the grids. In order to produce the skeleton of such a fuel assembly, the tubes are kept fixed to the end members and then the spacing grids are rigidly fixed to the thus fixed tubes. During the operation of fixing the guide tubes to the end members, it is necessary to draw out or stretch the said tubes with a certain predetermined force in order to ensure that the latter are in contact with the end member and thus permit the compensation of the statistical length differences which may exist at the time of manufacture between the various tubes. This fitting of the tubes and their joining to the end members is brought about by a screwed connection, which is a complicated, long operation involving the use of tools making it possible to measure the torques applied. Thus, these torques must be below a limit value, so as to prevent any deterioration of the threads. In order to manufacture such an assembly, the spacing grids are then rigidly fixed to the thus screwed tubes and it is then necessary to dismantle at least one of the end plates in order to be able to fit the fuel rods. This then requires the reinstallation of the plate or plates, whilst obviously taking the same precautions with regards to the torque applied when tightening each screw.
Unfortunately the screwing and tightening of threaded members for bringing about the fixing of the end members to the bundle of guide tubes has always led to a twisting effect in the tubes, due to the introduction of twisting torques. As it is in this state of the tubes that the various spacing grids are rigidly fixed thereto, said twisted state may then be fixed and retained subsequently, which may lead to a permanent geometrical defect of the assembly, which is prejudicial to its good behaviour in the pile, as well as to its strength and operating life.